Method for the rhodium-catalyzed hydroformylation of olefins with reduction of rhodium losses

ABSTRACT

Process for preparing aldehydes and alcohols by rhodium-catalyzed hydroformylation of olefins having 6–20 carbon atoms with subsequent separation by distillation of the output from the hydroformylation reaction into the hydroformylation products and a rhodium-containing solution and recirculation of this solution to the hydroformylation reaction, wherein the rhodium concentration of the recirculated rhodium-containing solution is 20–150 ppm by mass.

The present invention relates to a process for preparing aldehydes andalcohols by rhodium-catalyzed hydroformylation of olefins, in which thelosses of active rhodium catalyst occurring as a result of the work-upof the output from the hydroformylation reaction are minimized.

On an industrial scale, aldehydes and/or alcohols are prepared byhydroformylation of olefins using cobalt or rhodium catalysts. The useof rhodium catalysts is usually advantageous, since they enable higherselectivities and product yields to be achieved. However, rhodium isexpensive compared to cobalt, so that the catalyst is a notinsignificant cost factor in the hydroformylation of olefins to give thecorresponding aldehydes using rhodium catalysts. To improve theeconomics, the specific catalyst consumption therefore has to bereduced. This is the amount of catalyst which has to be introduced intothe process during long-term operation to ensure a constant activitylevel of the catalyst.

The rhodium-catalyst conversion of olefins into the correspondingaldehydes is usually carried out in a homogeneous, liquid phase. Inhydroformylations in a homogeneous phase, i.e. catalyst, olefins,products, solvents, etc., are present in one phase, there is the problemof separating the catalyst from the reaction mixture after the reaction.This can be achieved in a simple fashion by distilling off the unreactedstarting material and the products; the catalyst dissolved in thebottoms, usually in high-boiling components, is subsequently returned tothe reactor. The distillation can be carried out continuously orbatchwise.

Decomposition or deactivation of the catalyst is frequently observed inthe separation of the rhodium catalyst from the reaction mixture bydistillation. Particularly in the hydroformylation of relativelylong-chain olefins, the distillation can only be carried out at elevatedtemperature and/or under reduced pressure because of the high boilingpoints of the products, i.e. the conditions required for separating offthe catalyst promote its deactivation.

A number of methods of reducing catalyst deactivation during the work-upof the output from the reactor in rhodium-catalyzed hydroformylationreactions of olefins are known:

EP 0272608 describes a process in which a rhodium catalyst comprisingtriphenylphosphine oxide ligands is used. In the work-up of the reactionmixture, triphenylphosphine is added (in an amount nine times that ofthe rhodium present), prior to its distillation. The distillationresidue thus comprises rhodium complexes having triphenylphosphine asligands and also triphenylphosphine and triphenylphosphine oxide. Theentire, i.e. free and complex, triphenylphosphine is subsequentlyoxidized to triphenylphosphine oxide and this catalyst solution isreturned to the reactor. Oxygen or a peroxide is used for the oxidationof the triphenylphosphine. Further variants of this method aredescribed, for example, in JP 63 222 139, JP 63 208 540, DE 3 338 340and JP 63 218 640.

These processes have the disadvantage that triphenylphosphine iscontinually consumed. The equivalent amount of triphenylphosphine oxideis formed from it by oxidation. To limit the concentration of this inthe catalyst liquid or in the reactor, a purge stream is necessary, andthis in turn results in discharge of rhodium. In addition, an oxidationapparatus is necessary. Unless air is used for the oxidation, theoxidation incurs costs for the oxidant.

To stabilize the phosphorus-containing ligands used for the rhodium,further compounds can be introduced into the hydroformylation reactionand/or the work-up step for the reaction products.

U.S. Pat. Nos. 5,731,472, 5,767,321 and EP 0 149 894 describes processesfor the hydroformylation of n-butenes. These processes employ rhodiumcatalysts which contain phosphite ligands and are stabilized by additionof amines. A disadvantage is that amines can act as catalysts for aldolcondensations and thus promote the formation of high boilers.

The hydroformylation in the presence of rhodium complexes as catalystsof a C₈-olefin mixture which has been prepared by dimerization ofbutenes and the stabilization of the complexes by means of substitutedphenols are described in JP 04-164042. Here, rhodium compound, ligandand stabilizer are used in a molar ratio of 1/10/50. Disadvantages inthis process are the costs for the stabilizer and the expense ofseparating it off.

The rhodium catalyst is usually recovered as a solution in ahigh-boiling solvent by distillation of the output from thehydroformylation reaction under mild conditions.

The hydroformylation of olefins in the presence of rhodium catalystsfrequently forms small amounts of high boilers as by-products. The highboilers are aldolization and acetalization products and also, as aresult of disproportionation of the aldehydes into acids and alcohols,esters of these acids. In the distillation of the output from thehydroformylation, these high boilers remain in the distillation bottomstogether with the rhodium compounds. To enable the high boilerconcentration in the hydroformylation reactor to be kept at a constantvalue in a continuous process, an amount of high boilers correspondingto the amount formed, in the hydroformylation reaction has to beseparated off in the pseudo-steady state. This is achieved by controlleddischarge of part of the distillation bottoms. Proportionate amounts ofactive rhodium catalyst are also removed from the process circuit in thepurge stream. In the ideal case, the amount of rhodium discharged wouldcorrespond precisely to that amount of rhodium which would have to befed in in a continuous process in order to maintain a steady state.

However, in conventional processes, a larger amount of rhodium has to befed in because insoluble rhodium compounds, e.g. multinuclear rhodiumclusters, and/or metallic rhodium are formed during the work-up bydistillation as a result of, inter alia, the thermal stress. Althoughthis rhodium continues to be present in the apparatus, it iscatalytically inactive and is not available as catalyst in thehydroformylation reactor. Furthermore, the dissolved rhodium compoundspresent in the bottom product from the distillation, which are returnedto the hydroformylation reactor, are less active than a fresh rhodiumcatalyst, so that even at the same rhodium concentration in thehydroformylation reactor with recirculated rhodium compounds, a lowerspace-time yield is obtained.

The economics of a hydroformylation process using rhodium catalysts thusdepend mainly on the specific rhodium consumption. It is therefore anobject of the invention to develop a process for preparing aldehydes byrhodium-catalyzed hydroformylation of olefins which has a low catalystconsumption and can be carried out without addition ofcatalyst-stabilizing substances.

It has surprisingly been found that the catalyst consumption of arhodium-catalyzed process for the hydroformylation of olefins can begreatly reduced if particular rhodium concentrations in the distillationbottoms are not exceeded in the work-up of the output from thehydroformylation by distillation.

The present invention accordingly provides a process for preparingaldehydes and alcohols by rhodium-catalyzed hydroformylation of olefinshaving 6-20 carbon atoms with subsequent separation by distillation ofthe output from the hydroformylation reaction into the hydroformylationproducts and a rhodium-containing solution and recirculation of thissolution to the hydroformylation reaction, wherein the rhodiumconcentration of the recirculated rhodium-containing solution is 20–150ppm by mass.

The rhodium concentration in the recirculated rhodium-containingsolution is preferably 20–100 ppm by mass, in particular 20–50 ppm bymass, of rhodium.

To carry out the process of the invention, a number of variants arepossible:

FIG. 1 illustrates one variant of the process of the invention. Here,olefin and synthesis gas are fed via line 1 into the hydroformylationreactor 3, and catalyst solution is fed in via line 2. The output fromthe hydroformylation is conveyed via line 4 to the distillation stage 5where the desired aldehydes and alcohols are separated off as topproduct 6. The bottom product 7 comprises the rhodium catalyst and/orhigh boilers and/or aldehydes and alcohols which have not been separatedoff and/or inert solvents. Part of the rhodium-containing solution can,if appropriate, be discharged via line 8. The solution is recirculatedto the reactor 3, and fresh catalyst can be added via line 9.

Solvents present in the rhodium-containing solution can be the reactionproducts of the hydroformylation reaction, preferably aldehydes,alcohols and/or high-boilers formed and/or inert solvents such asTEXANOL, dioctyl phthalate (DOP) or diisononyl phthalate (DINP) whichhave been added. The rhodium concentration is set via the separation bydistillation of the output from the hydroformylation reaction, e.g. bymeans of an appropriate proportion of alcohols and aldehydes. The amountof bottom product can be set by means of appropriate operatingconditions of the distillation apparatus (temperature, pressure). Assolvents, it is possible to use the high boilers formed in the reactionor added inert solvents, in each case alone or in addition to thealcohols and aldehydes formed in the reaction. If high boilers are notformed in the reaction and no inert solvents are added, the alcohols andaldehydes alone can also serve as solvents.

Inert solvents which can be used are all compounds which have a boilingpoint sufficiently high to allow the desired aldehydes and alcohols tobe separated off and are inert in the hydroformylation reaction and thedistillation.

The starting materials for the process of the invention are olefins orolefin mixtures having from 6 to 20 carbon atoms and terminal and/orinternal C—C double bonds. The mixtures can comprise olefins having anidentical, similar (±2) or significantly different (>±2) number ofcarbon atoms. Examples of olefins, which can be used as startingmaterial either in pure form, in a mixture of isomers or in admixturewith further olefins having a different number of carbon atoms, are: 1-,2- or 3-hexene, 1-heptene, linear heptenes having an internal doublebond (2-heptene, 3-heptene, etc.), mixtures of linear heptenes, 2- or3-methyl-1-hexene, 1-octene, linear octenes having an internal doublebond, mixtures of linear octenes, 2- or 3-methylheptene, 1-nonene,linear nonenes having an internal double bond, mixtures of linearnonenes, 2-, 3- or 4-methyloctene, 1-, 2-, 3-, 4- or 5-decene,2-ethyl-1-octene, 1-dodecene, linear dodecenes having an internal doublebond, mixtures of linear dodecenes, 1-tetradecene, linear tetradeceneshaving an internal double bond, mixtures of linear tetradecenes,1-hexadecene, linear hexadecenes having an internal double bond,mixtures of linear hexadecenes. Further suitable starting materials are,inter alia, the mixture of isomeric hexenes formed in the dimerizationof propene (dipropene), the mixture of isomeric octenes formed in thedimerization of butenes (dibutene), the mixture of isomeric nonenesformed in the trimerization of propene (tripropene), the mixture ofisomeric dodecenes formed in the tetramerization of propene or thetrimerization of butenes (tetrapropene or tributene), the hexadecenemixture formed in the tetramerization of butenes (tetrabutene) and alsoolefin mixtures prepared by cooligomerization of olefins having adifferent number of carbon atoms (preferably from 2 to 4), ifappropriate after fractional distillation to give fractions having anidentical or similar (±2) number of carbon atoms. It is also possible touse olefins or olefin mixtures which have been produced by theFischer-Tropsch synthesis. In addition, olefins which have been preparedby olefin metathesis or by other industrial processes can be used.Preferred starting materials are mixtures of isomeric octenes, nonenes,dodecenes or hexadecenes, i.e. oligomers of lower olefins such asn-butenes, isobutene or propene. Other starting materials which arelikewise well suited are oligomers derived from C₅-olefins.

Modified rhodium complexes can be used as catalysts in the process ofthe invention. These rhodium catalysts can be introduced into theprocess in the form of their active complexes, but it is generallysimpler in industry to generate the active catalysts in situ fromstable, readily storable rhodium compounds. Rhodium compounds which aresuitable for this purpose are, for example, rhodium(II) and rhodium(III)salts such as rhodium(III) chloride, rhodium(III) nitrate, rhodium(III)sulfate, potassium rhodium sulfate, rhodium(II) or rhodium(III)carboxylates, rhodium(II) and rhodium(III) acetate, rhodium(II)octanoate, rhodium(II) nonanoate, rhodium(III) oxide, salts ofrhodic(III) acid, trisammonium hexachlororhodate(III). Also suitable arerhodium complexes such as dicarbonylrhodium acetylacetonate,acetylacetonatobis-ethylenerhodium(I) Particularly useful compounds arerhodium acetate, rhodium octanoate and rhodium nonanoate.

In general, about 1–500 and preferably 3–50 mol of ligand is used permole of rhodium. Fresh ligand can be added to the reaction at any pointin time to keep the concentration of free ligand constant.

In the process of the invention, the concentration of rhodium in thehydroformylation reactor is preferably from 1 to 50 ppm by mass, inparticular from 5 to 25 ppm by mass.

The choice of ligands added is not restricted in the process of theinvention, but depends on the olefin used and on the desired products.Preferred ligands are ligands containing nitrogen, phosphorus, arsenicor antimony atoms, and particular preference is given to phosphorusligands. The ligands can be monodentate or polydentate, and in the caseof chiral ligands, it is possible to use either the racemate or oneenantiomer or diastereomer. It is likewise possible to use a mixture oftwo or more different ligands. As phosphorus ligands, particularpreference is given to using those which form complexes with rhodiumwhich are less stable than the triphenylphosphine complexes, for examplephosphine oxides, phosphites, phosphonites and phosphinites.

Examples of phosphites are trimethyl phosphite, triethyl phosphite,tri-n-propyl phosphite, tri-i-propyl phosphite, tri-n-butyl phosphite,tri-i-butyl phosphite, tri-t-butyl phosphite, tris(2-ethylhexyl)phosphite, triphenyl phosphite, tris(2,4-di-t-butylphenyl) phosphite,tris(2-t-butyl-4-methoxyphenyl) phosphite,tris(2-t-butyl-4-methylphenyl), phosphite, tris(p-cresyl) phosphite.Further suitable phosphites are sterically hindered phosphite ligandswhich are described, for example, in EP 155 508, U.S. Pat. Nos.4,668,651, 4,748,261, 4,769,498, 4,774,361, 4,835,299, 4,885,401,5,059,710, 5,113,022, 5,179,055, 5,260,491, 5,264,616, 5,288,918,5,360,938, EP 472 071, EP 518 241 and WO 97/20795. Preference is givento using triphenyl phosphites which are substituted by 1 or 2 isopropyland/or tert-butyl groups on the phenyl rings, preferably in the orthoposition relative to the phosphite ester group. Examples of phosphonitesare methyldiethoxyphosphine, phenyldimethoxyphosphine,phenyldiphenoxyphosphine, 6-phenoxy-6H-dibenz[c,e][1,2]oxaphosphorin andtheir derivatives in which all or some of the hydrogen atoms arereplaced by alkyl or aryl radicals or halogen atoms, and also theligands described in the patents WO 9843935, JP 09-268152 and DE 198 10794 and in the German patent applications DE 199 54 721 and DE 199 54510.

Customary phosphinite ligands are described, for example, in U.S. Pat.No. 5,710,344, WO 95 06627, U.S. Pat. No. 5,360,938, JP 07082281.Examples are diphenyl(phenoxy)phosphine and its derivatives in which allor some of the hydrogen atoms are replaced by alkyl or aryl radicals orhalogen atoms, and diphenyl(methoxy)phosphine,diphenyl(ethoxy)phosphine, etc.

In the process of the invention, the rhodium-catalyst hydroformylationsare carried out at pressures of from 15 to 300 bar, preferably atpressures of from 15 to 270 bar, in particular at pressures of 150–270bar. The pressure employed depends on the structure of the startingolefins, the rhodium catalyst used and the desired effect. Thus, forexample, α-olefins can be converted into the corresponding aldehydes inhigh space-time yields at pressures below 64 bar. On the other hand, inthe case of olefins having internal double bonds, in particular branchedolefins, higher pressures are advantageous.

The temperatures for the rhodium-catalyzed, hydroformylations accordingto the invention are generally in the range from 40° C. to 180° C.,preferably from 90° C. to 160° C., in particular from 130 to 160° C.

After the hydroformylation, the major part of the synthesis gas isremoved by reducing the pressure. The catalyst is separated off from theliquid reaction output by distillation. The catalyst and any addedligands, stabilizers, etc., remain in/as the distillation residue.During start-up, or if only a small amount of high boilers is formed inthe process, it may be advantageous to use a high-boiling inert solvent(i.e. a solvent having a higher boiling point than the products andstarting materials) in which the catalyst dissolves. The catalystdissolved in the high-boiling solvent can then be recirculated directlyto the reactors. It is particularly advantageous to use the high-boilingby-products formed in the process as high-boiling solvent. Othersuitable solvents are high-boiling esters such as2,2,4-trimethylpentane-1,3-diol monoisobutyrate, which is commerciallyavailable as TEXANOL.

Various procedures can be employed industrially to separate off thecatalyst by distillation. The catalyst solution is preferably separatedoff by means of falling film evaporators, short-path evaporators or thinfilm evaporators or combinations of these apparatuses. The advantage ofsuch combinations can be, for example, that synthesis gas stilldissolved in the mixture and part of the products and the startingolefins still present can be separated off in a first step (for examplein a falling film evaporator) and the catalyst can then be separated offin a second step (for example in a thin film evaporator).

The distillation pressures are in the range from 5 mbar to 1 bar,preferably from 10 mbar to 100 mbar. The distillation temperatures arefrom 40 to 180° C., in particular from 80 to 150° C.

The rhodium-containing solution (bottom product) can, if desired, beadditionally stabilized with carbon monoxide, as described in DE 100 48301.1.

Part of the rhodium-containing solution (bottom product) can bedischarged to maintain a constant high boiler concentration in thehydroformylation reactor. The other part of the bottom product isrecirculated to the hydroformylation reactor. Part of the catalyst(rhodium and ligand) is removed from the process with the purge stream.These amounts and other deficits of rhodium and ligand in therecirculated stream have to be replaced to maintain a given catalystconcentration in the hydroformylation reactor. In the ideal case, onlythese amounts of rhodium catalyst need to be replaced.

If appropriate, further products can be separated off from the purgestream, for example by distillation.

The rhodium can be recovered from the purge stream by known methods.

The vapor obtained in the concentration step can be separated bydistillation into the target products, aldehydes and alcohols,hydrocarbons and other by-products. If appropriate, olefins can berecovered from the hydrocarbon fraction and recirculated to the process.

The aldehydes obtained by means of the process of the invention can beused as such, for example as fragrances, can be oxidized to carboxylicacids or can be hydrogenated to give alcohols.

The alcohols formed in the process of the invention or the alcoholsobtained by hydrogenation of the aldehydes are precursors for esters, inparticular plasticizers, for example phthalates, hydrophthalates,adipates, citrates, trimellitates, and for detergents. Furthermore, theycan be used as solvents.

The following examples illustrate the invention without restricting itsscope, which is defined by the description and the claims.

EXAMPLE 1

Rhodium-Catalyzed Hydroformylation of C12-Olefin (Tributene) Using FreshCatalyst

In a 10 l autoclave, 5000 g of tributene from the Octol process werereacted at 135° C. under a synthesis gas pressure of 250 bar in thepresence of a phosphite-modified rhodium catalyst for 4 hours. Theactive rhodium catalyst was generated in situ from rhodium octanoate andtris(2,4-di-tert-butylphenyl) phosphite. The rhodium concentration(based on the total reaction mixture) was set to 10 ppm, and the molarphosphorus/rhodium ratio (P/Rh) was 10:1.

As inert, high-boiling solvent, 250 g of TEXANOL(2,2,4-trimethylpentane-1,3-diol monoisobutyrate) were added to thereaction mixture.

The conversion of the olefin was monitored both by means of GC analysisand via the amount of synthesis gas taken up. After 4 hours, thereaction was stopped. The crude reaction product mixture comprised 16.5%by mass of C12-olefin, 78.1% by mass of C13-aldehyde (isotridecanal),4.3% by mass of C13-alcohol (isotridecanol) and 1.1% by mass of highboilers. This product composition corresponds to a tributene conversionof 81.2% and a yield of desired products (C13-aldehyde/-alcohol) of79.7%.

EXAMPLE 2 Comparative Example

Rhodium Recovery from the Crude Reaction Product Mixture

For the recovery of the catalyst, 2500 g of the crude reaction productmixture from Example 1 were fractionated in a laboratory thin filmevaporator at 130° C. and 20 mbar. In this procedure, therhodium-containing high boiler was separated off from the low boilers(desired products, unreacted C13-olefin). Under the separationconditions chosen, a rhodium-containing high boiler (bottom product)having a rhodium content of 248 ppm was obtained at the bottom of thethin film evaporator.

EXAMPLE 3 According to the Invention

Rhodium Recovery from the Crude Reaction Product Mixture

As in Example 2, 2500 g of the crude reaction product mixture fromExample 1 were treated in a laboratory thin film evaporator at 130° C.and 40 mbar to recover the catalyst. In this procedure, therhodium-containing high boiler was separated off from the low boilers(desired products, unreacted C13-olefin). Under the milder (compared toExample 2) separation conditions, a rhodium content of 43 ppm wasachieved in the bottom product.

EXAMPLE 4 Comparative Example

Hydroformylation of C12-Olefin (Tributene) Using Recycled Catalyst

In a 2 l autoclave, 1000 g of tributene from the Octol process werereacted at 135° C. under a synthesis gas pressure of 250 bar in thepresence of a recycled, used rhodium catalyst for 4 hours. As catalystprecursor, use was made of the highly concentrated rhodium-containinghigh boiler containing 248 ppm of rhodium obtained in Example 2. Therhodium-content (based on the total reaction mixture) was set to 10 ppm,as in Example 1. The molar phosphorus/rhodium ratio (P/Rh) was 10:1.

The conversion of the olefin was monitored both by means of GC analysisand via the amount of synthesis gas taken up. After a time of 4 hours, atributene conversion of 67.3% and a yield of desired product(C13-aldehyde/-alcohol) of 66.2% were determined. Compared to the use offresh rhodium catalyst (Example 1), a significant decrease in theconversion and the yield of desired product is found when the usedcatalyst is employed.

EXAMPLE 5 According to the Invention

Hydroformylation of C12-Olefin (Tributene) Using Recycled Catalyst

In a 2 l autoclave, 1000 g of tributene from the Octol process werereacted at 135° C. under a synthesis gas pressure of 250 bar in thepresence of a recycled, used rhodium catalyst for 4 hours. As catalystprecursor, use was made of the rhodium-containing high boiler fromExample 3, which at 43 ppm of rhodium had a lower rhodium concentrationthan that from Example 2. The rhodium content (based on the totalmixture) was set to 10 ppm, as in Examples 1 and 4. The molarphosphorus/rhodium ratio (P/Rh) was 10:1.

The conversion of the olefin was, as in Examples 1 and 3, monitored bothby means of GC analysis and via the amount of synthesis gas taken up.After a time of 4 hours, a tributene conversion of 80.5% and a yield ofdesired product (C13-aldehyde/-alcohol) of 79.0% were determined.Compared to the use of fresh catalyst (Example 1), no appreciabledecrease in the conversion and the yield of desired product is observedwhen the used catalyst prepared from less concentratedrhodium-containing high boilers containing 43 ppm of Rh is employed.

1. A process for preparing aldehydes and alcohols comprising: subjectingolefins having 6–20 carbon atoms to a rhodium-catalyzed hydroformylationat an initial concentration of rhodium, distilling the output from thehydroformylation, whereby hydroformylation products and arhodium-containing solution are separated from said output, whilesetting the rhodium concentration of the rhodium-containing solution to20–150 ppm by mass, and recirculating said rhodium-containing solution,whereby the concentration of rhodium in the recirculatedrhodium-containing solution is adjusted to an initial concentration ofrhodium.
 2. The process as claimed in claim 1, wherein therhodium-containing solution comprises the reaction products of thehydroformylation reaction as solvent and the recirculated rhodiumconcentration is set by means of the separation by distillation of theoutput from the hydroformylation reaction.
 3. The process as claimed inclaim 1, wherein the rhodium-containing solution comprises an inertsolvent as solvent and the recirculated rhodium concentration is set bymeans of the separation by distillation of the output from thehydroformylation reaction.
 4. The process as claimed in claim 1, whereinthe rhodium-containing solution comprises the high boilers, aldehydesand alcohols formed in the hydroformylation reaction as solvent and therecirculated rhodium concentration is set by means of the proportion ofaldehydes and alcohols via the separation by distillation of the outputfrom the hydroformylation reaction.
 5. The process as claimed in claim1, wherein the rhodium-containing solution comprises the aldehydes andalcohols formed in the hydroformylation reaction and an inert solvent assolvents and the recirculated rhodium concentration is set by means ofthe proportion of aldehydes and alcohols via the separation bydistillation of the output from the hydroformylation reaction.
 6. Theprocess as claimed in claim 5, wherein 2,2,4-trimethylpentane-1,3-diolmonoisobutyrate, dioctyl phthalate or diisononyl phthalate is used asinert solvent.
 7. The process as claimed in claim 1, wherein the rhodiumcatalysts comprise phosphite ligands.
 8. The process as claimed in claim7, wherein the rhodium catalysts comprise tris (2,4-di-t-butylphenyl)phosphite as ligand.
 9. The process as claimed in claim 2, wherein therhodium-containing solution comprises the high boilers, aldehydes andalcohols formed in the hydroformylation reaction as solvent and therecirculated rhodium concentration is set by means of the proportion ofaldehydes and alcohols via the separation by distillation of the outputfrom the hydroformylation reaction.
 10. The process as claimed in claim2, wherein the rhodium-containing solution comprises the aldehydes andalcohols formed in the hydroformylation reaction and an inert solvent assolvents and the recirculated rhodium concentration is set by means ofthe proportion of aldehydes and alcohols via the separation bydistillation of the output from the hydroformylation reaction.
 11. Theprocess as claimed in claim 3, wherein the rhodium-containing solutioncomprises the aldehydes and alcohols formed in the hydroformylationreaction and an inert solvent as solvents and the recirculated rhodiumconcentration is set by means of the proportion of aldehydes andalcohols via the separation by distillation of the output from thehydroformylation reaction.
 12. The process as claimed in claim 10,wherein 2,2,4-trimethylpentane-1,3-diol-monoisobutyrate, dioctylphthalate or diisononyl phthalate is used as inert solvent.
 13. Theprocess as claimed in claim 11, wherein2,2,4-trimethylpentane-1,3-diol-monoisobutyrate, dioctly phthalate ordiisononyl phthalate is used as the inert solvent.
 14. The process asclaimed in claim 2, wherein the rhodium catalysts comprise phosphiteligands.
 15. The process as claimed in claim 3, wherein the rhodiumcatalysts comprise phosphite ligands.
 16. The process as claimed inclaim 4, wherein the rhodium catalysts comprise phosphite ligands. 17.The process as claimed in claim 5, wherein the rhodium catalystscomprise phosphite ligands.
 18. The process as claimed in claim 6,wherein the rhodium catalysts comprise phosphite ligands.
 19. Theprocess as claimed in claim 1, wherein the rhodium concentration of therecirculated rhodium-containing solution is 20–100 ppm by mass.
 20. Theprocess as claimed in claim 1, wherein the rhodium concentration of therecirculated rhodium-containing solution 20–50 ppm by mass.
 21. Theprocess as claimed in claim 1, wherein the hydroformylation is carriedout at a pressure in the range of 150 to 270 bar.
 22. The process asclaimed in claim 1, wherein recirculation of the solution to thehydroformylation reaction is carried out without subjecting saidsolution to an oxidation.